1. Field of the Invention
The present invention relates to a semiconductor light-detecting device such as a phototransistor for converting an optical signal into an electrical signal.
2. Description of the Related Art
The research and development of digital optical communication techniques is intensifying because optical signal transmission is superior to electrical signal transmission with respect to transmission speed and elimination of interference between signals. However, a practical device capable of direct conversion of an optical signal into a digital electrical signal has not yet been developed. That is, the following processes must be performed: after an optical signal is converted into an electrical signal, the electrical signal must be amplified by an amplifier and then converted into a digital signal by an A/D (analog/digital) converter.
Photoconductors, photodiodes, avalanche photodiodes, phototransistors, and the like are conventionally used as photodetection devices.
The photoconductor is a device for extracting a current using an effect (photoconductive effect) in which an electric resistance is decreased by irradiation of light. An MSM (Metal Semiconductor Metal) photoconductor shown in FIG. 17 is widely known. In FIG. 17, reference numeral 71 denotes a semiconductor substrate, and reference numerals 72 denote electrodes (anode and cathode electrodes).
A photoconductor cannot have current gain, but can have a high quantum conversion efficiency. However, an increase in the quantum conversion efficiency unavoidably reduces the frequency bandwidth, or range, of the device. This is consequential to the requirement of space charge neutrality within the device. After pairs of electrons and holes are generated by incident light, when a voltage is applied to the photoconductor by an external circuit, the holes and electrons flow toward the anode and cathode electrodes, respectively. The traveling or transit time of the holes is longer than that of the electrons, especially in III-V group compound semiconductors. In order to maintain internal charge neutralization, electrons are injected continuously from the external circuit until the holes either are recombined with the electrons or reach the anode electrode.
The quantum conversion efficiency of the device is determined by the ratio of the mean transit time of holes to the mean transit time of electrons. By prolonging the mean transit time of holes, a high quantum conversion efficiency can be equivalently obtained. In this case, however, the frequency range of the device is disadvantageously narrowed. In this manner, the conventional photoconductor cannot simultaneously have a high quantum conversion efficiency and a wide frequency range.
FIG. 18 is a sectional view showing a conventional pin photodiode.
This pin photodiode has a structure obtained by stacking an n-type semiconductor layer 81, an intrinsic (i-type) semiconductor light-absorbing layer 82, and a p-type semiconductor layer 83. In FIG. 18, reference numerals 84 and 85 denote an anode electrode and a cathode electrode, respectively.
The pin photodiode does not have current gain, but has a frequency range wider than that of a conventional photoconductor. The quantum conversion efficiency of the pin photodiode cannot exceed 100%, and external gain is required to obtain a high voltage output.
In contrast to this, an avalanche photodiode has a frequency range almost equal to the frequency range of a conventional pin photodiode, and has current gain (.gtoreq.10). However, a power supply voltage sufficient to cause avalanche breakdown is required to obtain current gain, and a carrier multiplication time for obtaining an avalanche gain is required. For this reason, high gain results in decreased frequency range.
FIG. 19 is a sectional view showing a conventional phototransistor.
This phototransistor 90 has a structure in which a p-type semiconductor layer 92 is formed by diffusion in an n-type semiconductor light-absorbing collector layer 91, and an n-type semiconductor layer 93 is formed by diffusion in the p-type semiconductor layer 92. In FIG. 19, reference numerals 94, 95, and 96 denote an emitter electrode, a collector electrode, and a power supply, respectively.
A current flowing between the n-type semiconductor light-absorbing collector layer 91 and the n-type semiconductor layer 93 is controlled by injecting carriers generated within the n-type semiconductor light-absorbing collector layer 91 into the p-type semiconductor layer 92. Although the phototransistor can have a relatively high current gain (.gtoreq.100), the speed of the current is disadvantageously decreased due to a carrier accumulation effect of a base region, causing the frequency range of the device to be reduced. The device thus cannot simultaneously have high current gain and wide frequency range.
An output from the device is linear with respect to an optical input L.sub.IP, and an A/D conversion process must be still performed to obtain a digital output signal. Because the external circuit cannot easily be replaced with an electrical digital switch, when the device 90 is to be used, as shown in FIG. 20, after the optical input L.sub.IP is converted into an electrical signal, the electrical signal is amplified by an amplifier 97, and the amplified signal is converted into a digital output signal S.sub.OP by an A/D (analog/digital) converter 98.